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![[image of flower]](../../ima/flowXS_05.gif)
Matter |
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Chronology
Matter is anything that takes up space. Material is another word for matter. Mass is the measure of the amount of matter. A golf ball has more mass than a ping-pong ball, i.e., it has more matter. A substance is matter with a uniform and definite composition. Sugar is a substance because any sample contains exactly the same amount of sugar, but kool-aid, which contains sugar, water, and dye is not a substance because its composition varies slightly from container to container. Most substances can exist in any of 3 normal states, or a combination of them: solid, liquid, or gas. A solid has a definite shape and volume. A liquid flows, has a fixed volume, and takes the shape of its container. A gas takes the shape and volume of its container. Liquids and gases are called fluids because they flow. Solidification or freezing means changing from a liquid state to a solid state. Fusion or melting means changing from a solid state to a liquid state. Evaporation or vaporization means changing from a liquid state to a gas state below the liquid's normal boiling point. Such a gas is called a vapor. Moist air contains water vapor, since the water gas is below water's normal boiling point. Boiling means changing from a liquid state to a gas state at or above the boiling temperature. Steam is boiled water. Condensation or condensing means changing from a gas state to a liquid state. Sublimation means changing from a solid state directly to a gas state without the intermediate change from solid state to liquid state. A mixture is a blend of two or more substances. Since the blend varies in composition, a mixture itself is not a substance. There are 2 kinds of mixtures: heterogeneous and homogenous. A heterogeneous mixture is not uniform in compositon, e.g., soup, concrete. The individual substances in a heterogenous mixture can be identified. These mixtures also scatter light (Tyndall effect). A homogeneous mixture, also called a solution, has a uniform composition, e.g., air, tap water, gasoline. The individual substances in a solution cannot be identified and a solution will not scatter light (no Tyndall effect). Unlike a substance, both kinds of mixtures can vary in composition from sample to sample. There are 2 kinds of heterogenous
mixtures: suspensions and colloids. The particles in a suspension
settle out in time, whereas the particles in a colloid do not. Colloids, which are
very important in industry, are grouped by type: Notes: A gas in a gas forms a homogeneous, not a heterogeneous, mixture; that is, a solution. Some heterogenous mixtures, e.g., salad dressing and sand in water, exhibit properties of both a suspension and a colloid; some particles settle out, but others remain suspended. Substances are divided into two groups: elements and compounds. An element is the simplest form of matter that can exist under ordinary laboratory conditions, e.g., oxygen, iron, nitrogen, etc. Elements cannot be separated into into simpler substances by chemical reactions; they are the building blocks for all other substances. Elements combine chemically to form compounds. Therefore, a compound, e.g., water, calcium carbonate, carbon dioxide, etc. is a substance that can be separated chemically into simpler substances by a chemical reaction. An atom is the smallest particle of an element that retains the properties of that element. An atom is made up of many subparticles. The main ones are protons, neutrons, and electrons. Protons are positively charged particles. Electrons are negatively charged particles. Neutrons have no charge. Protons and neutrons, which are about the same mass, are bound together to form an atom nucleus. Nucleon refers to either a proton or a neutron. Electrons move in orbitals around the nucleus. Under normal conditions, the number of protons and electrons, which have the same magnitude of charge, are the same, so that the atom is electrically neutral. However, if an atom gains or loses one or more electrons, it becomes electrically charged. An electrically charged atom is called an ion. An atom that has lost one or more electrons has a positive charge. It is called a positive ion or a cation. An atom that has gained one or more electrons has a negative charge. It is called an anion. Although atoms of any one element have a fixed number of protons and electrons, they usually contain different numbers of neutrons and, therefore, have different masses. The mass number of an atom is the number of protons plus the number of neutrons. Each of these different mass numbers of one atom are called an isotope. A nuclide usually refers to an isotope in general, not necessarily to those of a particular atom, although the two words are often used interchangeably. Although the mass numbers of any one isotope is a whole number, the weighted average of all isotopes of one atom used in chemical calculations is a whole number plus a fraction. Electron mass is only 1/283 of a nucleon, so it is ignored in mass calculations. In molecular compounds, atoms are bonded together to form molecules. A molecule is an electrically neutral group of atoms acting as a unit. All molecules in a compound are the same. Molecules in different compounds are different. In ionic compounds, cations and anions join together. It also is electrically neutral. Molecules can gain or lose electrons to become molecular ions (actually, "formula units", since the definition of the word "molecule" is restricted to an unionized compound). For example, when ammonia, which is electrically neutral, loses an electron, it becomes the ammonium ion, with a 1+ charge, a cation. The word particle is used to include atoms, ions, and molecules. A fourth state of matter is called a plasma. A plasma is a gaseous mixture of electrons and cations. Many atoms of elements can radiate particles and/or an electromagnetic field, either naturally or artificially. For all atoms of all 116 elements, there are 279 stable (non-radiating) isotopes and about 2700 unstable (radiating) isotopes. The particle radiation may be in the form of alpha particles, beta particles. Electromagnetic radiation is usually either gamma ray or X-rays. The alpha particles (alpha rays) are two protons and two neutrons bound together into highly ionized particles that are identical to a helium nucleus, i.e., He2+. Alpha particles are emitted by radioactive nuclei such as uranium or radium in a process called alpha decay. Alpha rays are easily absorbed by materials and can travel only a few centimeters in air. They can be absorbed by thin paper or the outer layers of human skin and so are not generally dangerous to life unless the source is ingested or inhaled. If alpha radiation enters the body in large doses, it causes radiation poisoning. Wiki n.p. Beta particles (beta rays) are high-energy electrons (or positrons). They have more energy than alpha particles and can be stopped by a thin sheet of metal. Potassium-40 is one example of an isotope that emits beta particles. Beta particles travel ten times farther, but have one-tenth the ionizing power of alpha particles, so large doses are not likely to be harmful. When electrons travel from a hot cathode toward an anode of high voltage in a tube evacuated of air, (cathode-ray tube, vacuum tube, Crookes tube) and deflected by a magnetic field onto a phosphorescent screen, they are called cathode rays. A history of discoveries about matter is given below. To 1790 In 440 BCE, the Greek philosopher, Leucippus, speculated that all matter was composed of elementary particles he called atoms. His theory did not pass peer review. Asimov 47 |
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